[CentOS] File retrieval from outside hangs, internally is okay, only Centos5 affected

Miskell, Craig Craig.Miskell at agresearch.co.nz
Sun Oct 7 19:49:54 UTC 2007

> I have a really weird problem with some of my servers, namely all the
> ones running Centos5
> When I try to download a file from the server to a machine outside our
> Cisco 6500 router/firewall, the download hangs about half the times
> (15 out of 40) when less than half a megabyte into the transfer
> (varied from 76 kb to 496 kb).
> One server has a portchannel (Cisco speak for ethernet bundle), others
> do not, some use e1000, some use broadcom, I've tried httpd and scp
> transfers, I've tried from three different clients,
> Here's the really annoying part: the problem only occurs when
> downloading from outside the firewall, not when transferring files
> internally in the serverroom! And the switch is the firewall is a
> modular chassis, so the data comes over the same backplane regardless.
> And there's 42 files in /proc/net/sys/ipv4 which differ between EL4
> and Centos5, so I'm a little lost here
> Anybody got some ideas?
Just to state the obvious (well, obvious to me), which you don't seem to
have mentioned above: The filtering part of the 6500 is dropping the
traffic, and is dropping it because of something that Centos 5 is doing
differently from EL4.  I think there was a post to this list just last
week about something similar; I don't have time to search the archive,
but it is something to do with a TCP option/extension which is on in
Centos 5, but can be turned off via a setting in /proc somewhere; the
extension should be acceptable to all firewalls/routers (uses a
previously unused few bits in the TCP header), but some decide it's not
valid and drop packets/connections.

Craig Miskell
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